In the case of integrated semiconductor memories, a distinction is made between volatile semiconductor memories and nonvolatile semiconductor memories. Nonvolatile semiconductor memories store the items of information even after the operating voltage has been switched off. Nonvolatile semiconductor memories may be EEPROMs (electrically erasable programmable read only memories), for example. By contrast, volatile semiconductor memories store the items of information only during their operation until the operating voltage is switched off. Even during operation, stored items of information continually have to be refreshed again since they are otherwise lost through leakage currents despite the operating voltage being maintained. Thus, by way of example, memory cells of DRAMS (dynamic random access memories) are in each case read and rewritten in amplified fashion (refreshing) after a few or a few dozen milliseconds, since it is only during this refresh time that error-free storage of the items of information is ensured.
Volatile memory cells have the advantage that they require little substrate area and can be arranged densely on a semiconductor chip. Memory cells of DRAMs, for instance, can be realized just with a storage capacitor, for example in the form of a trench capacitor or a stacked capacitor, and a selection transistor connected to in each case a word line and a bit line.
Volatile memory cells and nonvolatile memory cells are usually accommodated on different semiconductor chips. Although there is the need to permanently store certain items of information in the case of nonvolatile semiconductor memories, too, storing these items of information would necessitate nonvolatile memory cells on the memory chip of the volatile semiconductor memory, which can conventionally be realized at best by an additional memory cell array for nonvolatile memory cells or by a very small number of fuses or antifuses arranged outside the cell array of the volatile memory cells. In the case of an additional memory cell array for nonvolatile memory cells, however, it is necessary to effect dedicated driving of the additional memory cell array by means of word lines and bit lines including the corresponding drivers and the other electrical control circuits. On account of the disproportionately large additional amount of area taken up on the semiconductor substrate, such an additional nonvolatile memory cell array on a memory chip for the volatile semiconductor memory is dispensed with in most cases. Instead, only the most necessary items of information, namely usually only the identification designator of the respective semiconductor chip, are stored with the aid of laser fuses on the volatile memory chip. By contrast, other items of information, for instance the test results originating on account of an electrical functional test carried out with the aid of the volatile memory device, for instance test errors that occurred or the positions or numbers of replaced word or bit lines or test parameters such as temperatures, current values, trimming values, burn-in test time durations or the like, are not stored on the fabricated and then tested semiconductor chip, even though this would actually be desirable.
Consequently, there is a requirement for integrated volatile semiconductor memories that additionally afford the possibility of storing a relatively large quantity of data permanently, that is to say in nonvolatile fashion. In particular, there is a requirement for a memory cell that in conjunction with little additional occupancy in terms of substrate area being taken up, enables nonvolatile storage of as many digital items of information of the semiconductor memory as possible.